1. Field of the Invention
The present invention relates to magnetoresistive (MR) sensor biasing. More particularly, the present invention relates to magnetoresistive sensor biasing methods and circuits.
2. Description of the Related Art
For the purpose of this disclosure, the terms "magnetoresistive (MR) sensor" and "MR head" both encompass any magnetoresistive element used for detecting a varying magnetic field by detecting a change in resistance .DELTA.R.sub.S of the element resistance R.sub.S caused by variations in the magnetic field. Also for purposes of this disclosure, the terms MR sensor and MR head are both not limited to magnetic recording elements only. Examples of embodiments of MR sensors that are contemplated to be within the scope of these two terms include, but are not limited to, barber pole MRs, dual MRs, soft adjacent layer MRs, differential MRs, spin valve MRs, tunnel junction MRs and giant MRs.
An MR sensor is an energetically passive sensor that uses a sense current (or voltage) for detecting a change in sensor resistance in the form of a signal voltage or signal current. The sense current may also be used for providing all or part of a magnetic bias for the sensor that is needed for proper MR sensor operation. This disclosure does not distinguish between sense and bias current (or voltage). The terms (MR sensor) bias current and voltage are used interchangeably throughout this disclosure. The biasing and readout electronics associated with an MR sensor are generally referred to as an arm electronic (AE) module.
The physical characteristics of an MR sensor are determined by the properties of the materials used, the MR sensor sandwich configuration, and the sensor dimensions, that is, the thickness, height and length of the sensor. The dimension having the greatest tolerance in externally exposed MR sensors used for magnetic recording is the sensor height h. The tolerance on this dimension is for compensating for head wear and lapping for tape heads, and for ABS lapping for disk drive heads.
FIG. 1(a) shows the physical relationships between the different resistances forming the total MR head resistance R.sub.H. FIG. 1(b) shows a schematic diagram for an electrical model of resistance R.sub.H. The total MR head resistance R.sub.H is measured at terminals 10 and 11 of the AE module. The MR head and the AE module are shown as part of a disk drive 12 in FIG. 1(a). The portion of the total MR head resistance that varies in accordance with a varying magnetic field is the sensor resistance R.sub.S, with the resulting change in resistance being indicated as .DELTA.R.sub.S. Resistance R.sub.S varies inversely proportional to the sensor height h. The total head lead resistance R.sub.l is the resistance of the wires to the pre-amplifier of the AE module and the resistance of the back-leads BL inside the MR head. Lastly, the total front-lead resistance of the MR head is indicated as R.sub.f. Resistance R.sub.f also varies inversely proportional to the sensor height h. However, R.sub.f shows no signal variations for varying magnetic field. Equations (1)-(4) represent these principles in symbolic notation. EQU R.sub.H =R.sub.l +R.sub.f +R.sub.S (1) EQU R.sub.f, R.sub.S .varies.1/h, (2) EQU h=sensor height, (3)
and EQU R.sub.S =R.sub.S0 .+-..DELTA.R.sub.S (signal induced). (4)
In FIGS. 1(a) and 1(b) and throughout the following disclosure, bias current I.sub.B equals head bias current I.sub.H. The voltage V.sub.H is the bias voltage appearing across the total MR head resistance R.sub.H. Voltage V.sub.B is the bias voltage appearing across sensor resistance R.sub.S and front lead resistance R.sub.f. Voltage V.sub.S is the bias voltage appearing across sensor resistance R.sub.S. Height h is the physical height of the sensor.
There are three conventional approaches for providing MR sensor biasing: a constant bias current scheme; a constant bias voltage scheme; and an adjusted bias current scheme for attaining constant bias voltage. The term "constant" as used when referring to biasing schemes indicates a biasing invariability from sensor to sensor in view of manufacturing tolerances. For a conventional constant bias current scheme, the same bias/sense DC current is applied to all MR heads by AE modules during manufacturing of a product regardless of the respective head resistances, sensor heights, etc., of the MR heads. FIGS. 2(a) and 2(b) illustrate an example of biasing conditions for an MR head for different sensor resistances R.sub.H for a conventional constant bias current I.sub.B scheme. If I.sub.B in FIG. 2(a) is 10 mA, for example, FIG. 2(b) shows that the head bias voltage V.sub.H varies accordingly between 200 mV and 500 mV as R.sub.H varies between 20 and 50 Ohms.
For a conventional constant bias voltage scheme, the same bias/sense DC voltage is applied to all MR heads in a product by an AE module. FIGS. 2(c) and 2(d) illustrate exemplary bias conditions for an MR head for different sensor resistances R.sub.H for a conventional constant bias voltage V.sub.H scheme. If the bias voltage V.sub.B appearing across the sensor resistance R.sub.S in FIG. 2(d) is 500 mV, for example, FIG. 2(c) shows that the bias current I.sub.H through the MR head varies inversely proportional to the total head resistance R.sub.H.
U.S. Pat. No. 5,309,294 to Cahalan, issued May 3, 1994, discloses voltage biasing circuit that provides a constant bias voltage for an MR head. According to Cahalan, voltage biasing circuit includes a nulling circuit that adjusts the output of the voltage biasing circuit. The nulling circuit effectively increases the output of the voltage biasing circuit by an amount that is approximately equal to a cable drop across any parasitic cable resistances that are present. The Cahalan circuit uses a resistive element having a resistance that is an estimate of the parasitic cable resistance for generating the nulling voltage. This approach, however, does not compensate for any voltage drop appearing across the back-leads inside an MR head.
In the conventional adjusted bias current scheme for attaining a constant bias voltage, the bias current through the MR head is adjusted during manufacturing to be inversely proportional to the MR head resistance R.sub.H, achieving an essentially constant MR head bias voltage. The resistance R.sub.H must be measured during the manufacturing process.
None of these conventional MR head biasing scheme compensate for variations in physical characteristics of the MR head that occur during the manufacturing process, specifically, the sensor dimension tolerances. Consequently, operation of an MR head biased by a conventional biasing scheme is typically not at or near the operational optimum for the head. Further, uniform magnetic performance from MR head to MR head of a product, that is, less variability in magnetic performance from head to head regardless of manufacturing tolerances, is not typically achieved.